Existing equipment and methods for ion implantation process wafers either singly or in batches. Batch processing is usually performed on spinning disks which pass the wafers through the ion beam. Most ion implanters used for serially processing 200 or 300 mm diameter single wafers operate by scanning the ion beam in one direction. Others ion implanters operate by passing the wafer through a continuous parallel uniform ribbon beam. Some recent commercial equipment utilizes a scheme for scanning a wafer in two dimensions in a raster pattern through a stationary ion beam. Various operational and structural details about these different kinds of ion implanters and systems is described by U.S. Pat. Nos. 4,234,797; 4,922,106; 4,980,562; 4,276,477; 5,834,786; 6,313,474; 5,003,183; 5,046,148; 5,180,918; and 6,313,484. The text of each cited patent, as well as its internally cited publications, is expressly incorporated by reference herein.
The Single Wafer Hybrid Ion Implantation Technique
Of particular interest is the conventionally known single wafer hybrid ion implantation technique in which a wafer is mechanically scanned in at least one direction through a ribbon-shaped ion beam. The ribbon beam may be scanned at a higher frequency in an orthogonal direction; or alternatively, the wafer is moved through a continuous ion beam of sufficient width that scanning is not required.
Prior to reaching the workpiece, the ion beam usually passes through a focusing device (e.g., a dipole magnet or electrostatic lens) whose focus coincides with the center of the beam scanning device, or the point of origin of a continuous beam, thereby rendering the beam trajectories substantially parallel.
For a single wafer system, the wafer mechanical scan usually consists of multiple passes of the wafer at constant velocity through the ribbon ion beam. During each pass, a constant velocity is maintained until one edge of the wafer has substantially cleared the location of the ribbon beam. Then, the wafer velocity is rapidly decelerated to zero; and then accelerated to a constant velocity in the opposite direction for a subsequent pass of the wafer through the ribbon ion beam. In the manner, the wafer is passed multiple times in opposite directions through the ribbon ion beam until the desired dose of ions has been implanted into the wafer.
Prior Art FIG. 1 shows conventionally known hybrid scan single wafer equipment in which a silicon wafer is mechanically scanned in a vertical direction through an ion beam, and which is electrostatically scanned in a horizontal direction. In operation, the ion beam is scanned so that it is always parallel to a fixed axis within a fraction of one degree. The wafer is scanned by mounting it on a holder attached to a vertical shaft which runs through air bearings. The scan velocity is typically up to 25 cm per second. It is desirable to minimize any variation in the incident angle of the ion beam at different parts of the wafer surface. It is also required to control and vary this incident angle by providing one or more tilt axes.
Such conventionally known implanter equipment is required to process over 100 wafers per hour, and sometimes processes over 300 wafers per hour; and each wafer process may involve several passes of the wafer through an ion beam. Therefore, over a typical ten-year lifetime use for the equipment, the scan mechanism may operate in excess of 10 million cycles.
The Equipment Problems Encountered in Single Wafer Hybrid Ion Implantation
The difficulties in engineering durable equipment for performing single wafer hybrid ion implantation include: (a) the need to minimize the mass of the moving assembly while retaining stiffness; (b) the need to provide a reliable seal between the vacuum in which the process occurs and the atmosphere; (c) the need to provide a means of tilting the wafer with respect to the beam axis; and (d) simultaneously, the need to keep the mechanism simple, reliable, and accessible for service.
However, moving mechanisms in vacuum are difficult to lubricate, and suffer from more rapid wear than those in atmosphere. Furthermore, lubricants are potential contaminants of the process; and friction in the vacuum will generate particles which can contaminate the wafer. It would be desirable to minimize or even eliminate friction within the vacuum; and to locate all bearings and other mechanisms involving friction, potential wear, and intermittent contact, in the atmosphere—where they are isolated from the process and where conventional methods of lubrication and maintenance may be freely used.
In addition, air bearings are reliable but expensive. The mass of the air bearing assembly's moving parts is high, making it more difficult to rapidly decelerate and accelerate the assembly in order to maximize the efficiency of the mechanical scanning.
The Other, Alternative Types of Ion Implanters Conventionally Available
Batch ion implanters can and do successfully provide uniform ion dosing of silicon wafers, even though the velocity of a wafer on a spinning disk varies with its radial coordinate on the disk. The ion uniformity is accomplished by varying either (i) the velocity with which either the disk is translated through the ion beam, or (ii) the velocity with which the ion beam is translated across the spinning disk—such that the velocity is proportional to 1/R, where R is the radial coordinate of the beam centroid on the disk relative to the spin axis. This approach is used in the prior art hybrid scan batch system illustrated in Prior Art FIG. 2, in which wafers are mounted on spinning disks, the beam is magnetically scanned, and the 1/R scan dependence is generated by means of the magnetic scan waveform.
Prior Art FIG. 3 shows the equipment disclosed by U.S. Pat. No. 5,834,786 in which a continuous ribbon beam is used, and wafers are mounted on a spinning disk. In this instance, the required radial dependence is present in the intensity profile along the long dimension of the ribbon beam, which is created by suitably shaped poles in the dipole analyzing magnet.
Also among the conventionally available assemblies and techniques for controlling the current density uniformity of ion beams are the following:
(A) The invention disclosed by U.S. Pat. No. 5,350,926 teaches the use of magnets for analyzing, shaping and rendering parallel an ion beam as well as the use of multipole elements (either integrated into bending magnets or as separate assemblies) for controlling the uniformity of the beam.
(B) In a commercial implanter sold by Mitsui Engineering and Shipbuilding (the MDI-100), a discrete multipole device is presented as a rectangular array of iron pole pieces mounted on a yoke which surrounds the ion beam. Each pole piece is individually excited by a separate coil wound around it. The resulting magnetic field is applied in the central rectangular aperture, through which the ribbon-shaped ion beam passes; and consists of spatially varying field components, which cause a local slight deflection of the trajectories for the ions passing through it. See for example, U.S. Pat. Nos. 5,834,786 and 5,350,926 for additional details of this arrangement.
(C) Prior Art FIG. 4 shows the equipment of Nogami et al, disclosed by U.S. Pat. No. 5,003,181,in which the wafer chuck is mounted on a radial arm and the chuck rotation motor is used to counter-rotate the wafer (axis F in Prior Art FIG. 4)—such that its motion has a uniform projection of velocity normal to the ribbon beam. This equipment, and the method of controlling it, suffer certain disadvantages which the present invention is intended to avoid, viz:                (i) The rotation mechanism which counter-rotates the wafer operates continuously at high duty cycle. It is required to have great precision and low backlash.        (ii) The motion from the implant location to the load position is a complex long motion requiring modest accelerations to ensure that the wafer is securely retained, thereby limiting overall throughput of the implant system        (iii) The load position is not sufficiently remote from the beam and limits the space for instrumentation        (iv) The required rotary motion involves active deceleration and acceleration during the passage of the wafer through the beam, in order to accomplish uniform projection of velocity on a straight line. This unfortunately introduces a high risk of backlash-generated velocity errors.        
(D) In other recent prior art equipment, a beam spot smaller than the wafer is used, and the wafer is passed mechanically through the beam in a two-dimensional raster pattern. This involves high accelerations but does make possible uniform implantation with little or no systematic variation between the ion beam and the wafer surface. It can be reliably used at very low energies and high currents, should control of the uniformity of a ribbon beam prove too difficult to accomplish.